To define the mechanisms for transport of secretory (s) IgA into bronchial secretions, IgA and secretory component in human bronchial mucosa were localized by immunoelectron microscopy IgA was identified in plasma cells located near glandular epithelial cells and on the basolateral plasmalemma and endocytic vesciles of the epithelial cells, especially of mucous cells. Secretory component was localized to the perinuclear spaces, endoplasmic reticulum, Golgi complexes, basolateral plasmalemma and endocytic vesicles of glandular epithelial cells, and those of ciliated epithelial cells. These findings are consistent with a model for IgA transport in which IgA dimers, synthesized in plasma cells, are complexed with secretory component on the basolateral plasmalemma of epithelial cells in bronchial glands and transport across the cells in endocytic vesicles to the gland lumina.